A typical concrete reinforcing tendon assembly will include a pair of anchor members which are in spaced-apart relation and have an elongated reinforcing tendon mounted between the tendon anchorages. The tendon will be placed under an axial load, either by pretensioning or post-tensioning, and secured in the tensioned condition by the anchorages. Tensioning of the tendon after formation and setting of the concrete structure in which the assembly is mounted is known as post-tensioning and is used widely in the formation of prestressed concrete structures.
The tendon can take the form of a single wire, strand or bar, secured by the anchorages, or a plurality of these reinforcing elements. It is also common for a concrete structure to include both prestressed and non-prestressed reinforcing elements, and typically these various elements cross and are in metal-to-metal contact with each other.
The presence of metallic concrete reinforcing tendon assemblies in concrete structures creates a substantial potential corrosion problem. Moreover, metal-to-metal contact can, under certain circumstances, create the passage of stray electrical currents through the steel tendon elements as a result of potential differentials that may exist between various parts of the concrete structure. These stray currents, together with any undesirable substances in the concrete that may come into contact with the steel have been known to cause failures due to corrosion, stress corrosion or hydrogen embrittlement of the steel.
Under adverse environmental conditions, for example, use of the concrete structure in sea water, the chloride ions in the water afford an electrolyte that forms a corrosion cell locally or between inter-connected reinforcement. Thus, if the concrete is not impervious enough to the sea water or has cracks large enough to permit the penetration of chloride ions to the steel, and if oxygen is available, corrosion will occur.
The corrosion of steel forms products of corrosion which have a volume of about 10 times the volume of the steel from which the corrosion originated. This creates a large expansive force at the corrosion site that most concretes cannot withstand, and concrete cracking and spalling occurs. Spalling and cracking of the concrete will further lead to extensive corrosion as further steel is exposed in the structure.
Current practice in the concrete post-tensioning industry is to protect post-tensioned concrete reinforcing assemblies from corrosion in one of two distinctive ways. First, an unbonded tendon assembly construction can be employed in which the tendon is coated with a corrosion inhibiting grease-like product and is encased in a sheath or conduit. In the case of a single bar, strand or wire, the sheath or conduit is typically a plastic tube. When the tendon assembly includes a tendon formed by a multiplicity of side-by-side wires, strands or bars, the sheath or conduit is normally a metal duct which is filled with the grease-like corrosion inhibitor. The assembly is referred to as "unbonded" since the corrosion-inhibiting grease is not capable of supporting any of the axial load intermediate of the two tendon anchorages.
The second corrosion-inhibiting practice commonly employed is the use of a bonded tendon assembly structure. The elongated tendon in a bonded construction is encased in a sheath into which a cement grout is injected after the tendon has been tensioned and anchored. When the grout inside the conduit sets, it is capable of transferring the axial tension force in the tendon to the conduit or sheath and from there to the concrete structure. Thus, the tendon is, in effect, bonded to the concrete structure by the corrosion-inhibiting grout inside the sheath.
In both the unbonded and the bonded approaches the protective conduit or sheath around the elongated reinforcing tendon is connected at its ends to the anchorage or to a trumpet which extends from the anchor plate of the anchorage. The trumpet, anchor plate and anchorage assembly, however, are usually formed of steel and, further, are usually mounted in pockets or recesses in the concrete which are later filled with cement mortar or grout after the tendon has been tensioned.
While both the bonded and unbonded corrosion-inhibiting techniques employed in the concrete post-tensioning industry effectively reduce the incidence of corrosion, they do not eliminate corrosion, since the resulting structure includes substantial metallic elements which are exposed to the concrete and its inherent porosity and possibility for cracking. Even when a bonded or unbonded post-tensioned reinforcing assembly is used in combination with epoxy-coated unstressed reinforcing bars or the like, there is still a potential for corrosion.
Another approach to the corrosion problem in connection with reinforced concrete has been attempts to decrease concrete porosity by improved concrete consolidation. Thus, the use of extensive vibration and centrifugal casting to consolidate the concrete in order to reduce porosity and the entrapment of air has been found to be effective in preventing the penetration of chloride ions or similar corrosive materials in the concrete to the steel. Additionally, the thickness or depth at which the steel is recessed from the outside of the concrete structure will also affect corrosion. As the thickness increases, it is less likely that chloride ions will penetrate to the steel for any given concrete porosity. Unfortunately, however, increases in concrete thickness are accompanied by dramatic increases in the weight of the structure, and often by an increased need for reinforcing steel.
Typically, for normal concrete construction a cover of less than about 51 millimeters (2 inches) will tend to experience more corrosion problems. If the concrete is consolidated by centrifugal casting, special vibration, dry mix shotcreting or the like, the coverage can be reduced to about 19 millimeters (3/4 inch) or, in some instances, even as low as 12 millimeters (1/2 inch). These vibration, special casting and similar techniques, however, are often accompanied by an undesirable increase in cost or are not susceptible for use with many types of structures.